Dye-sensitized solar cell including a porous insulation substrate and a method for producing the porous insulation substrate

ABSTRACT

The present invention relates to a dye-sensitized solar cell including a working electrode (1), a first conducting layer (3) for extracting photo-generated electrons from the working electrode, a porous insulation substrate (4) made of a microfibers, wherein the first conducting layer is a porous conducting layer formed on one side of the porous insulation substrate, a counter electrode including a second conducting layer (2) arranged on the opposite side of the porous substrate, and electrolyte for transferring electrons from the counter electrode to the working electrode. The porous insulation substrate comprises a layer (5) of woven microfibers and a layer (6) of non-woven microfibers disposed on the layer of woven microfibers. The present invention also relates to a method for producing a dye-sensitized solar cell.

FIELD OF THE INVENTION

The present invention relates to a dye-sensitized solar cell including aporous insulation substrate made of microfibers, having a firstconducting layer formed on one side of the porous insulation substrate,and a second conducting layer arranged on the opposite side of theporous substrate. The present invention further relates to a porousinsulation substrate for a dye-sensitized solar cell. The presentinvention also relates to a method for producing the porous insulationsubstrate and the conducting layers.

PRIOR ART

Dye-sensitized solar cells (DSC) have been under development for thelast 20 years and work on similar principles as photosynthesis. Unlikesilicon solar cells, these cells obtain energy from sunlight using dyeswhich can be manufactured cheap, environmentally unobtrusive and inabundance.

A conventional sandwich type dye-sensitized solar cell has a few μmthick porous TiO₂ electrode layer deposited onto a transparentconducting substrate. The TiO₂ electrode comprises interconnected TiO₂metal oxide particles dyed by adsorbing dye molecules on the surface ofthe TiO₂ particles and forming a working electrode. The transparentconducting substrate is normally a transparent conducting oxidedeposited onto a glass substrate. The transparent conducting oxide layerserves the function as a back contact extracting photo-generatedelectrons from the working electrode. The TiO₂ electrode is in contactwith an electrolyte and another transparent conducting substrate, i.e. acounter electrode.

Sunlight is harvested by the dye, producing photo-excited electrons thatare injected into the conduction band of the TiO₂ particles and furthercollected by the conducting substrate. At the same time, I⁻ ions in theredox electrolyte reduce the oxidized dye and transport the generatedelectron acceptors species to the counter electrode. The two conductingsubstrates are sealed at the edges in order to protect the DSC modulesagainst the surrounding atmosphere, and to prevent the evaporation orleakage of the DSC components inside the cell.

WO 2011/096154 discloses a sandwich type DCS module including a porousinsulation substrate, a working electrode including a porous conductingmetal layer formed on top of the porous insulation substrate andcreating a back contact, and a porous semiconductor layer containing anadsorbed dye arranged on top of the porous conducting metal layer, atransparent substrate facing the porous semiconductor layer, adapted toface the sun and to transmit the sun light to the porous semiconductorlayer. The DSC module further includes a counter electrode including aconducting substrate arranged on a side opposite to the poroussemiconductor layer of the porous insulation substrate, and at adistance from the porous insulation substrate, thereby forming a spacebetween the porous insulation substrate and the conducting substrate. Anelectrolyte is filled in the space between the working electrode and thecounter electrode. The porous conducting metal layer may be createdusing a paste including metallic or metal based particles, which isapplied on top of the porous insulation substrate by printing, andfollowed by heating, drying and baking. An advantage with this type ofDSC module is that the conducting layer of the working electrode isarranged between the porous insulation substrate and the poroussemiconductor layer. Thus, the conducting layer of the working cell doesnot have to be transparent, and can be made of a material of highconductivity, which increases the current-handling capability of the DSCmodule and ensures high efficiency of the DSC module.

There are high demands on the porous insulation substrate. An idealporous insulation substrate must fulfill the following requirements:

The substrate must have sufficient mechanical strength to withstand themechanical handling and processing. During the processing of the DSC thesubstrate is subjected to mechanical handling such as: cuttingprocesses, stacking and de-stacking processes, printing processes,drying processes, air/vacuum sintering processes, sealing processes,etc. Substrates with poor mechanical strength can suffer damage duringhandling and processing, resulting in defect solar cells, which lowerthe manufacturing yield.

The substrate must have sufficient high temperature resistance andexhibit low mechanical deformation and/or small loss in mechanicalstability after high temperature treatment. During processing thesubstrate is subjected to temperatures of 500° C. in air and (580-650)°C. in vacuum or inert atmosphere. The substrate must withstandtemperatures in air up to 500° C. without significant mechanicaldeformation or loss in mechanical stability. The substrate mustwithstand temperatures in vacuum or inert atmosphere of at least up to580° C. or higher without significant mechanical deformation or loss inmechanical stability.

The substrate must be chemically inert to high temperature processing.During the various high temperature treatments the substrate is exposedto, e.g., hot air, hot air containing organic solvents, hot aircontaining organic combustion products and to hydrogen gas. Thesubstrate must be chemically inert to all these high temperaturetreatments and not react chemically to produce compounds that could beharmful for the DSC.

The substrate must withstand the chemicals used in the DSC. The DSCcontains active substances such as, e.g., organic solvents, organicdyes, and ions such as I⁻ and I³⁻ etc. In order to have a goodperformance stability and life time of the DSC the substrate must notreact with the active substances of the DSC to alter the chemicalcomposition of the DSC or produce compounds that could be harmful forthe DSC.

The substrate must allow for fast transport of ions between theelectrodes. In order to have fast ion transport between the electrodes,the substrate must have sufficiently high porosity (pore volumefraction) and low tortuosity.

The substrate has to be electrically insulating. This is to preventelectrical short circuit between the counter electrode and the currentcollector.

The distance between the counter electrode and the working electrode isaffected by the thickness of the substrate. The distance between thecounter electrode and the working electrode should be as small aspossible such that the transport of ions between the counter electrodeand working electrode is as fast as possible. Therefore, the thicknessof the substrate should be as thin as possible.

The substrate must have sufficient capacity to block the conductiveparticles in the printing ink from seeping through the substrate. Inorder to avoid electrical short circuit between conducting layersprinted on both sides of the substrate, the substrate must be able toblock the conductive particles printed on one side of the substrate fromseeping through to the other side of the substrate.

To summarize, the porous insulation substrate must allow ions to passthrough the substrate and prevent particles to pass through thesubstrate, and must have sufficient mechanical properties.

In WO 2011/096154 it is proposed to use a molded fiber glass compact asthe porous insulation substrate. The molded fiber glass compact can bewoven glass fabric containing glass fibers, or non-woven fiberglass inthe form of a sheet having glass fibers, which are joined by suitablemeans.

By using high temperature compatible glass based substrates it ispossible to fulfill most of the above requirements. However, if thesubstrate is made of non-woven microglass fibers, the substrate has tobe made very thick in order to withstand the mechanical handling andprocessing during manufacturing of the solar cell. This is due to thefact that non-woven glass microfibers have very poor mechanicalproperties, and accordingly, substrates based on non-woven glassmicrofibers must be produced with very high thicknesses in order toincrease their mechanical stability. A substrate with high thicknessleads to a large distance between the counter electrode and the workingelectrode, and accordingly, to a very slow transport of ions between thecounter electrode and working electrode.

Woven glass fibers, i.e. glass fabric, include woven yarns of glassmicrofibers, where each glass fiber yarn consists of multiple glassmicrofibers. Woven glass fibers are inherently mechanically strongercompared to non-woven glass fibers. Additionally, the thickness of wovenfibers can be made very thin with maintained mechanical strength.However, woven fibers often have large holes between the woven yarns,which cause a large amount of particles in printed inks to pass rightthrough the substrate in an uncontrolled way across the entire area ofthe woven fiber causing electrical short circuit between the counterelectrode and current collector. Thus, the holes in the fabric make itdifficult to apply an ink including metallic or metal based particles onboth sides of the porous insulation substrate without creating anelectrical short-circuit, unless the particles are much larger than theholes. However, having such large particles in the ink makes theconducting metal layers too thick. Thick conducting metal layers willincrease the distance between the counter electrode and the workingelectrode resulting in a slower ion transport between the counterelectrode and the working electrode.

OBJECT AND SUMMARY OF THE INVENTION

The object of the present invention is to provide a dye-sensitized solarcell having a porous insulation substrate that fulfills theabove-mentioned requirements.

This object is achieved with a dye-sensitized solar cell as definedherein.

The dye-sensitized solar cell includes a working electrode, a firstconducting layer for extracting photo-generated electrons from theworking electrode, a porous insulation substrate made of microfibers,wherein the first conducting layer is a porous conducting layer formedon one side of the porous insulation substrate, a counter electrodeincluding a second conducting layer arranged on the opposite side of theporous substrate, and electrolyte for transferring electrons from thecounter electrode to the working electrode. The solar cell ischaracterized in that the porous insulation substrate comprises a layerof woven microfibers and a layer of non-woven microfibers disposed onthe layer of woven microfibers on a first side of the substrate.

A microfiber is a fiber having a diameter less than 10 μm and largerthan 1 nm.

We have found that by combining the properties of woven and non-wovenmicrofibers, it is possible to achieve all the above requirements for anideal porous insulation substrate. A woven fabric can be made very thinand mechanically very strong, but it contains large holes between thewoven yarns. On the other hand, the non-woven microfiber is mechanicallyweak, but has excellent filtering properties that blocks conductiveparticles in the printing ink from seeping through the porous insulationsubstrate. By depositing a thin layer of non-woven microfibers on top ofa layer of woven microfibers, it is possible to prevent the particles inthe inks from passing right through the woven fiber, and it is possibleto achieve all the above requirements. The thin fragile layer ofnon-woven microfibers is mechanically stabilized by the supporting layerof woven microfibers.

According to an embodiment of the invention, the first conducting layeris disposed on the layer of non-woven microfibers. The non-woven layerprovides a smooth surface on the substrate, suitable for applying asmooth conducting layer on the substrate by printing.

According to an embodiment of the invention, the layer of wovenmicrofibers comprises yarns with holes formed between the individualwoven yarns, and at least a part of the non-woven microfibers areaccumulated in the holes between the yarns. Thus, the thickness of thelayer of non-woven microfibers varies in dependence of the locations ofthe holes in the woven layer of microfibers, such that the layer ofnon-woven microfibers is thicker in the holes in the layer of wovenmicrofibers and thinner on top of the yarns of layer of wovenmicrofibers. The layer of non-woven microfibers protrudes into the holesbetween the yarns. This embodiment reduces the thickness of the layer ofnon-woven microfibers and makes it possible to provide a thin substrate.Thereby, the distance between the counter electrode and the workingelectrode becomes small and the transport of ions between the counterelectrode and working electrode becomes fast. The thickness of thesubstrate becomes significantly reduced compared to providing auniformly thick layer of non-woven microfibers on top of a sheet ofwoven fibers, such as stacking a sheet of non-woven fibers on top of asheet of woven fibers.

According to an embodiment of the invention, the porous insulationsubstrate comprises a second layer of non-woven microfibers arranged onthe layer of woven microfibers on a second side of the substrate. Byproviding a second layer of non-woven microfibers on the other side ofthe layer of woven microfibers, a symmetrical and more mechanicallystable substrate is achieved, and the substrate is prevented fromcurling during the heat treatment during the manufacturing of the solarcell. Additionally, the second layer of non-woven microfibers furtherenhances the blocking of conductive particles in the inks from passingright through the woven fibers. This embodiment provides a smoothsurface on both sides of the substrate and thus makes it possible toapply smooth conducting layers on both sides of the substrate byprinting. Preferably, the second conducting layer is disposed on thesecond side of the substrate on the second layer of non-wovenmicrofibers.

According to an embodiment of the invention, the layer of wovenmicrofibers is made of woven yarns including a plurality of microfibers,in the following denoted filaments, and the diameter of the microfibersin the layer of non-woven microfibers is smaller than the diameter ofthe filaments in the layer of woven microfibers. This embodiment enablesthe fibers to accumulate in the holes between the yarns and thus blockthe holes.

According to an embodiment of the invention, the layer of wovenmicrofibers is made of ceramic microfibers, such as a glass fabric.Ceramic microfibers are mechanically very strong and can be made verythin and still be strong enough. Ceramic microfibers can also withstandthe high temperatures used in the heat treatment of the solar cellduring the manufacturing procedure. Ceramic microfibers are fibers madeof a refractory and inert material, such as glass, silica (SiO₂),alumina (Al₂O₃), aluminosilicate and quartz.

According to an embodiment of the invention, the layer of non-wovenmicrofibers is made of ceramic microfibers, such as non-woven glassmicrofibers. The ceramic microfibers can withstand the high temperaturesused in the heat treatment of the solar cell during the manufacturingprocedure.

According to an embodiment of the invention, the thickness of the layerof woven microfibers is between 4 μm and 30 μm, preferably between 4 μmand 20 μm and more preferably between 4 μm and 10 μm. Such a layerprovides the required mechanical strength at the same time as it is thinenough to enable a fast transport of ions between the counter electrodeand working electrode.

According to an embodiment of the invention, the microfibers in thelayer of non-woven microfibers have a diameter of less than 4 μm,preferably less than 1 μm, and more preferably less than 0.5 μm. The useof very thin fibers reduces the thickness of the layer of non-wovenmicrofibers and accordingly the thickness of the substrate. Further, thethin fibers efficiently block the holes in the layer of wovenmicrofibers and prevent conductive particles from seeping through thesubstrate and thus prevent the formation of an electrical short circuit.

A further object of the present invention is to provide a porousinsulation substrate that fulfills the above mentioned requirements.This object is achieved with a porous insulation substrate. The porousinsulation substrate comprises a layer of woven microfibers and a layerof non-woven microfibers disposed on the layer of woven microfibers.Preferably, the woven microfibers are made of ceramic microfibers. Thefurther features described above related to the porous insulationsubstrate of the solar cell are also applicable to the porous insulationsubstrate itself.

According to an embodiment of the invention, the layer of wovenmicrofiber and the layer of non-woven microfibers are made of ceramicmicrofibers, such as glass microfibers. Ceramic microfibers aremechanically very strong and can be made very thin and still be strongenough.

According to another embodiment of the invention, the layer of non-wovenmicrofibers comprises organic microfibers. Organic microfibers arefibers made of organic materials, such as polymers, for example,polycaprolactone, PET, or PEO, and cellulose, for example nanocellulose(MFC) or wood pulp. It is possible to use organic microfibers in thelayer of non-woven microfibers. Organic microfibers cannot withstand thehigh temperatures used in the heat treatment during manufacturing of adye sensitized solar cell. However, organic microfibers can serve thepurpose of blocking the conductive particles in the inks from seepingright through the woven fibers during printing and drying of the inks onthe porous insulating substrate, thereby reducing the risk of electricalshort circuit. The organic microfibers are then removed during heattreatment at higher temperatures. Organic fibers are more flexible andnot as fragile as ceramic fibers. Thus, by adding organic fibers, themechanical strength of substrate increases, which for example isadvantageous during a printing and drying process.

According to a further embodiment of the invention, the layer ofnon-woven microfibers comprises organic microfibers and ceramicmicrofibers. The layer of non-woven microfibers is made of organic andceramic microfibers. An advantage of mixing organic microfibers andceramic microfibers in the layer of non-woven microfibers is that theorganic microfibers are thinner than the ceramic microfibers, therebycreating a nano-network of organic fibers inside a micro network ofceramic fibers and by that reducing the size of the holes in the micronetwork. The organic fibers fill up the holes between the microfibersthereby improving the ability to block the particles in the ink and thusavoiding short circuit. Further, by mixing organic microfibers andceramic microfibers in the layer of non-woven microfibers, themechanical strength of the substrate is improved compared to only havingceramic microfibers in the substrate.

Another object of the present invention is to provide a method forproducing a porous insulation substrate that fulfills the abovementioned requirements and a porous conducting layer formed on theinsulation substrate.

This object is achieved by a method as defined herein.

The method comprises:

-   -   a) producing the porous insulation substrate by providing a        fabric of woven microfibers comprising yarns with holes formed        between them, preparing a fiberstock solution by mixing liquid        and microfibers, covering a first side of the fabric with the        fiberstock solution, draining liquid from the fiberstock        solution through the holes in the fabric, and drying the wet        fabric with the microfibers disposed on the fabric, and    -   b) depositing an ink comprising conductive particles on one side        of the insulation substrate to form a porous conducting layer.

By draining the liquid from the fiberstock solution through the holes inthe fabric, the microfibers follow the liquid and a main part of thenon-woven microfibers are accumulated in the holes between the yarns,and accordingly, the size of the holes between the yarns is reduced.This method makes it possible to manufacture an insulation substrate,which is compact enough to prevent the conductive particles in the inkfrom passing through the substrate and thin enough to allow a fasttransport of ions between the counter electrode and working electrode.The layer of non-woven fibers on top of the layer of woven fibersprovides a smooth surface to print.

According to an embodiment of the invention, the fabric is made of wovenceramic microfibers, and said fiberstock solution is prepared by mixingliquid and ceramic microfibers.

According to an embodiment of the invention, the fiberstock solution isprepared by mixing liquid and organic microfibers.

According to an embodiment of the invention, the fiberstock solution isprepared by mixing liquid, ceramic microfibers, and organic microfibers.

The ink is deposited on top of the disposed microfibers to form a porousconducting layer on a first side of a porous insulation substrate.According to an embodiment of the invention, step a) further comprisescovering a second side of the fabric with the fiberstock solution, anddraining the liquid from the fiberstock solution through the holes inthe fabric, and step b) further comprises: depositing the ink on thesecond side of the fabric on top of the disposed microfibers, to form aporous conducting layer on a second side of the porous insulationsubstrate. This embodiment provides a smooth surface on both sides ofthe substrate and thus makes it possible to apply smooth conductinglayers on both sides of the substrate by printing.

According to an embodiment of the invention, step a) further comprisesadding a binder to the fiberstock solution. The addition of a binder tothe fiberstock solution enhances the binding of non-woven fibers to eachother and enhances the binding of non-woven fibers to the fabric.Further, adding a binder to the fiberstock solution makes it possible toreduce the amount of fiber added to the solution to achieve asatisfactory coverage of the holes in the fabric. Examples of bindersare, e.g., polyvinyl alcohol (PVA), starch, carboxymethyl cellulose(CMC) and nanocellulose, i.e., so called microfibrillated cellulose(MFC).

According to an embodiment of the invention, the method furthercomprises adding one or more additives selected from a group including asurfactant, a dispersant, a wetting agent, a defoamer, a retention aid,and a rheology changing agent, to the fiberstock solution. By usingadditives, it is possible to manufacture a thinner and denser substratewith smaller holes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained more closely by the description ofdifferent embodiments of the invention and with reference to theappended figures.

FIG. 1 shows a cross section through a dye-sensitized solar cell moduleaccording to an embodiment of the invention.

FIG. 2 shows an optical microscope picture of a glass fabric.

FIG. 3 shows an optical microscope picture of a glass fabric treatedwith 20 g glass microfiber stock solution on both sides.

FIG. 4 shows an optical microscope picture of a glass fabric treatedwith 80 g glass microfiber stock solution on both sides.

FIG. 5 shows a cross section through a porous insulation substrateaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention will now be explained more closely by the description ofdifferent embodiments of the invention and with reference to theappended figures. FIG. 1 shows a cross section through a dye-sensitizedsolar cell (DSC) according to an embodiment of the invention. The DSCdisclosed in FIG. 1 is of a monolithic type. The DSC comprises a workingelectrode 1 and a counter electrode 2. The space between the workingelectrode and the counter electrode is filled with an electrolyteincluding ions for transferring electrons from the counter electrode tothe working electrode. The DSC module comprises a conducting layer 3 forextracting photo-generated electrons from the working electrode 1. Theconducting layer 3 serves as a back contact and is in the followingnamed the back contact layer. The working electrode 1 includes a porousTiO₂ electrode layer disposed onto the back contact layer 3. The TiO₂electrode comprises TiO₂ particles dyed by adsorbing dye molecules onthe surface of the TiO₂ particles. The working electrode is positionedon a top side of the DCS module. The top side should be facing the sunto allow the sunlight to hit the dye molecules of the working electrode.

The DSC module further includes a porous insulation substrate 4 arrangedbetween the working electrode 1 and the counter electrode 2. Theporosity of the porous insulation substrate will enable ionic transportthrough the substrate. For example, the porous insulation substrate 4 ismade of a ceramic microfiber, such as glass microfibers. Substrates madeof ceramic microfibers are electrical insulators, but are porous andthereby allowing liquids and electrolyte ions to penetrate. The ceramicmicrofibers are cheap, chemically inert, can withstand high temperaturesand are simple to handle in various process steps.

The porous insulation substrate 4 comprises a layer of woven microfibers5 and a first layer of non-woven microfibers 6 disposed on the layer ofwoven microfibers 5 on a first side of the substrate. This makes itpossible to provide a thin and strong substrate. The back contact layer3 is a porous conducting layer disposed on the first side of thesubstrate on the layer of non-woven microfibers 6. In the embodimentdisclosed in FIG. 1, the substrate further comprises a second layer ofnon-woven microfibers 7 disposed on the layer of woven microfibers 5 ona second side of the substrate. By providing layers of non-wovenmicrofibers on both sides of the layer of woven microfibers, asymmetrical substrate is achieved. This may prevent the substrate fromcurling during the heat treatment during the manufacturing of the solarcell, and additionally contributes to prevent the particles in theprinted ink to pass through the layer of woven microfibers. The porousinsulation substrate 4 will be described in more detail below withreference to FIG. 5.

The counter electrode includes a conducting layer 2, in the followingnamed the counter electrode layer. In this embodiment, the conductinglayer 2 is a porous conducting layer disposed on the second side of theporous insulation substrate 4 on top of the second layer of non-wovenmicrofibers 7. When a porous conducting layer is used as a counterelectrode, it is part of the counter electrode opposite to the workingelectrode. The back contact layer 3 and the counter electrode layer 2are separated physically and electrically by the porous insulationsubstrate 4. However, the back contact layer and the counter electrodelayer are electrically connected via ions penetrating the porousinsulation substrate. The porous conducting layers 2,3 may be createdusing an ink including metallic or metal based particles, which areapplied on top of the porous insulation layer 4 by printing, andfollowed by heating, drying and baking. The particles are typicallybetween 0.1-10 μm. preferably between 0.5-2 μm.

The DSC also includes a first sheet 8 covering a top side of the DSCmodule and a second sheet 9 covering a bottom side of the DSC module andacting as barriers in order to protect the DSC modules against thesurrounding atmosphere, and to prevent the evaporation or leakage of theDSC components inside the cell. The first sheet 8 on the top side DSCmodule covers the working electrode and needs to be transparent,allowing light to pass through.

A thinner porous substrate is better, since a small distance between theworking electrode and the counter electrode provides minimal losses indiffusion resistance of the electrolyte. However, if the substrate istoo thin the mechanical strength of the substrate will be too low.Preferably, the thickness of the porous insulation substrate is largerthan 4 μm and less than 100 μm. More preferably, the thickness of theporous insulation substrate is less than 50 μm. The thickness of theporous insulation substrate is typically between 10-30 μm.

In the following, an example of porous insulating substrate according tothe invention will be described in more details. The porous insulatesubstrate is based on a layer of glass fabric made of woven yarnincluding a plurality of glass fibers. Woven fibers are much strongerthan non-woven fibers. Additionally, a layer of woven fibers can be thinwith maintained mechanical strength.

FIG. 2 shows an optical microscope picture of 15 μm thin glass fabric(Asahi Kasei E-materials). As can be seen in the figure, the glassfabric comprises woven yarn 10 a-b of glass fibers. Each yarn includes aplurality of glass fibers, also denoted filaments. The diameter of afilament is typically 4-5 μm, and the number of filaments in the yarn istypically 50. The glass fabric has large holes 14 between the wovenyarns, which would allow a large amount of the conductive particles inthe printed ink to pass right through the woven fiber in an uncontrolledway. This is an unwanted effect. The size of the holes can be as largeas 200 μm. In order to block the holes in the fabric, non-woven glassfibers are disposed on top of the fabric. This can be done by soakingthe fabric in a solution containing glass fibers and then removing theliquid part of the solution.

FIG. 3 shows an optical microscope picture of the glass fabric shown inFIG. 2 treated with 20 gram glass microfiber stock solution on bothsides, corresponding to 0.04 milligrams of deposited glass fiber persquare centimeter on each side. As can be seen in the figure, the wovenyarn in the glass fabric is covered by the disposed non-woven glassfibers. It can also be seen from FIG. 3 that the size of the holes inthe fabric is reduced. However, full coverage of the holes in the glassfabric is not accomplished.

FIG. 4 shows an optical microscope picture of the glass fabric shown inFIG. 2 treated with 80 gram glass microfiber stock solution on bothsides, corresponding to 0.16 milligrams of deposited glass fiber persquare centimeter on each side. As shown from FIG. 4, the holes are nowcovered by the glass microfibers. Obviously, full coverage of holes inthe glass fabric can be achieved by increasing the amount of glassmicrofiber. Thus, by depositing non-woven glass fibers on top of thewoven glass fibers it is possible to prevent that particles in theprinted inks pass right through the woven fibers.

If a binder such as, e.g., inorganic binders such as, silicates,colloidal silica particles, silanes (e.g., linear silane or branchedsilane or cyclic silane), and colloidal Al₂O₃ is added to the fiberstocksolution containing the glass fibers, the non-woven glass fibers canstick stronger to the woven fibers. Additionally, the layer consistingof deposited non-woven will be stronger mechanically as such.Consequently, by adding a binder to the fiberstock solution it ispossible to form a mechanically strong non-woven layer that adheresstrongly to the woven glass fibers.

EXAMPLE 1

In the following an example of a method for producing the poroussubstrate shown in FIG. 4 will be described. A 15 μm thin glass fabric(Asahi Kasei E-materials), as shown in FIG. 2, with 50 filaments, with afilament diameter of 4 μm, was laid on top of a stainless steel wirescreen (33 cm×33 cm) in a hand sheet former and a stock cylinder was puton top of the glass fabric and then closed and tightened. A glassmicrofiber stock solution was prepared by mixing 4000 grams of distilledwater and 8 grams of glass microfibers (Johns Manville, special purposetype glass microfiber type 90, fiber diameter: 0.2 μm) and 400 grams ofwater based colloidal silica (a solution containing around 15 wt. % SiO2in water) such that the final silica concentration was 1.4 wt. %. Themixing was performed using an Ultraturrax batch dispenser. The stockcylinder of the hand sheet former machine was filled with distilledwater (containing 1.4 wt. % silica) up to a level of 350 mm above thesurface of the wire screen. In the next step 80 grams of glassmicrofiber stock was poured into the hand sheet former machine. Theglass fiber stock and the distilled water containing silica were mixedby compressed air for 4 seconds and then allowed to settle for 6seconds, after which the water was drained through the glass fabric andthe wire screen. The wet treated glass fabric was dried at 110° C. inair in a belt oven. The glass fabric was then treated on the other sideusing the same process parameters as in the first treatment. Theresulting substrate is shown in FIG. 4. As can be seen in FIG. 4, thewoven yarn in the glass fabric is fully covered by the disposednon-woven glass microfibers. The thickness of the glass fabric withdisposed glass microfibers was around 30 μm. This means that the totalthickness of the two layers of non-woven microfibers is about 15 μm. Byusing a thinner glass fabric, it is possible to further reduce thethickness of the insulation substrate.

EXAMPLE 2

A variation of Example 1 is that the microfiber stock solution isprepared by mixing 4000 grams of distilled water and 200 grams ofnanocellulose dispersion (water based nanocellulose dispersioncontaining 2% by weight of nanocellulose) and 400 grams of water basedcolloidal silica (a solution containing 15 wt. % SiO2 in water). Thus,the ceramic glass microfibers in the microfiber stock solution arereplaced by organic microfibers consisting of nanocellulose. Usingnano-cellulose simplifies the manufacturing process in that dipping canbe used instead of using a paper manufacturing process.

EXAMPLE 3

Another variation of example 1 is that the microfiber stock solution isprepared by mixing 4000 grams of distilled water and 2 grams of glassmicrofibers (Johns Manville, special purpose type glass microfiber type90, fiber diameter: 0.2 μm) and 200 grams of nanocellulose dispersion(water based nanocellulose dispersion containing 2% by weight ofnanocellulose) and 400 grams of water based colloidal silica (a solutioncontaining 15 wt. % SiO2 in water). Thus, both organic microfibersconsisting of nanocellulose and ceramic microfibers consisting of glassare used in the microfiber stock solution. After the porous insulationsubstrate has been dried, ink with conductive particles is deposited onat least one side of the substrate on top of the layer of non-wovenmicrofibers, to form a porous conducting layer on the porous insulationsubstrate. If a monolithic DCS module is to be manufactured, the ink isdeposited on both sides of the substrate on top the layers of non-wovenmicrofibers, to form a porous conducting layer on each side of theporous insulation substrate. However, if a sandwich type DCS module isto be manufactured, the ink with conductive particles is only depositedon one side of the substrate.

To make sure that the fibers in the microfiber stock solution isproperly dispersed it is advantageous to add additives to the distilledwater before mixing water and the microfibers. Examples of suitableadditives are surfactants, dispersants, wetting agents, retention aids,defoamers, and rheology changing agents. It is advantageous to add oneor more of those additives. The additives are burnt away during thefollowing steps of the manufacturing process of the solar cell, andconsequently do not remain in the end product. The purpose of theadditives is to achieve individual and non-agglomerated fibers, so thatthe individual fibers can be deposited as homogeneously as possible inorder to provide a thin and at the same time dense layer of individualfibers. Thus, by using additives, it is possible to manufacture athinner and denser substrate with smaller holes.

By adding surfactants to the fiberstock solution and to the dilutionwater, a smoother and more homogeneous microfiber deposition can beaccomplished. Further, it is advantageous to add a wetting agent to thefiberstock solution so that the dilution water wets the fibers and thefabric. Also, by adding a water soluble polymer to the fiberstocksolution and the dilution water, a smoother and more homogeneousmicrofiber deposition can be accomplished. However, it was found that,when adding polymer it was necessary to add a defoaming agent in orderto avoid excessive foaming during dilution water filling and agitationand draining cycles. It is also advantageous to add rheology changingadditives to change the viscosity of the fiberstock solution and thedilution water.

It is also possible to add binders to the fiberstock solution and thedilution water to enhance the binding of non-woven fibers to each otherand to enhance the binding of non-woven fibers to the fabric. Bindersthat can be used are e.g. inorganic binders such as, silicates,colloidal silica particles, silane, e.g. linear silane, branched silane,or cyclic silane, and colloidal Al₂O₃.

It is also possible to add retention aids to the fiberstock solution andthe dilution water to improve the retention of the fibers in the porousinsulation substrate as it is being formed. Nanocellulose can be used asa retention aid.

FIG. 5 shows a cross section through a porous insulation substrate 4manufactured according to the method described in the example describedabove. The substrate has a layer 5 of woven microfibers including wovenyarns 10 comprising a plurality of filaments 11 and holes 14 formedbetween the yarns 10. The woven yarns 10 are preferably made of ceramicmicrofibers. The substrate also includes two layers 6, 7 of non-wovenmicrofibers arranged on each side of the layer 5 of woven microfibers.The layers 6, 7 of non-woven microfibers can be made of ceramicmicrofibers, organic microfibers or a combination thereof. As can beseen from the figure, a main part of the non-woven microfibers areaccumulated in the holes 14 between the yarns 10. This is a consequenceof the fact that the liquid from the fiberstock solution is drainedthrough the holes formed in the fabric. This leads to that the thicknessof the non-woven layers 6, 7 of microfibers varies in dependence on ofthe locations of the holes 14 in the woven layer of microfibers, suchthat the non-woven layer is thicker in the holes 14 in the woven layerand thinner on top of the yarns 17 of the woven layer. The side of thenon-woven layer 6, 7 that faces away from the woven layer 5 is smooth,but the opposite side of the non-woven layer that faces against thewoven layer is uneven and has thick parts 16 that protrude into theholes 14 of the woven layer and thin parts 17, which is are disposed ontop of the yarns 10. The present invention can be used for monolithic aswell as sandwich types of DCS.

The non-woven microfibers should preferably be thinner than thefilaments in the layer of woven microfibers. Thus, if the diameter ofthe filaments is about 4 μm, the fibers in the layer of non-wovenmicrofibers should have a diameter less than 4 μm, preferably less than1 μm, and more preferably less than 0.5 μm in order to block the holesin an efficient way. The length of the non-woven fibers is, for example100 nm-3 mm. For example, the diameter of nano-cellulose fibers istypically 5-10 nm and the length of the fibers is typically several μm.However, there also exist nano-cellulose fibers having a diameter of10-20 nm and a length of several mm.

The present invention is not limited to the embodiments disclosed butmay be varied and modified within the scope of the following claims. Forexample, the microfiber stock solution may include microfibers ofdifferent materials and diameters. Although, the examples above useglass microfibers, the invention is not limited to glass microfibers. Itis possible to use other types of ceramic microfibers with similarproperties. Further, the microfibers in the non-woven layer can be madeof a different ceramic material than the microfibers in the woven layer.Further, the microfibers in the non-woven layer can be made of organicmicrofiber such as cellulose or polymer.

In an alternative embodiment, the substrate may include a layer ofnon-woven microfibers and a layer of woven microfibers laminatedtogether.

In an alternative embodiment, the substrate has only one layer ofnon-woven microfibers, arranged on one side of a layer of wovenmicrofibers. Although it is advantageous to have non-woven layers onboth sides of the woven layer, it is not necessary. It is possible todeposit conducting layers on both sides of the substrate although onlyone of the sides of the woven layer has been provided with a layer ofnon-woven microfibers. The conducting layer can be printed on thenon-woven layer as well as on the woven layer. A substrate havingnon-woven layers deposited on both sides of the woven layer can becovered with a conducting layer on one side as well as on both sides.

In an alternative embodiment, the porous insulation substrate has onlyone layer of non-woven microfibers, arranged on one side of a layer ofwoven microfibers and the conducting layer is deposited on the otherside of the woven microfibers, i.e. the conducting layer is deposited onthe woven microfibers and not on the non-woven microfibers.

The porous insulating substrate is a porous and chemically inert andhigh temperature resistant and electrically insulating material that canbe used for other applications than in dye-sensitized solar cells. Thesubstrate can be used in filtering/filter applications for removing,e.g. dust, organic or inorganic or biological micro particles, flour,sand, smoke, bacteria, and pollen.

The substrate can also be used as a separator, materially separating thecathode and anode in electrochemical- or photoelectrochemical devicessuch as fuel cells, batteries, electrochemical sensors, electrochromicdisplays, and photoelectrochemical solar cells.

The invention claimed is:
 1. A solar cell comprising a porouselectrically-insulating substrate (4) made of microfibers, and at leastone conducting layer (3) comprising conductive particles, wherein theporous insulation substrate (4) is configured to block conductiveparticles printed on one side of the substrate (4) from seeping throughto the other side of the substrate (4) and comprises a sheet (5) ofnonconductive woven microfibers and a layer (6) of nonconductivenon-woven microfibers disposed on the sheet (5) of woven microfibers ona first side of the sheet (5), the layer (5) of woven microfiberscomprises yarns of woven microfibers separated by holes (14)therebetween, the layer (6) of non-woven microfibers has parts (16)deposited in and protruding into said holes (14) between the yarns ofthe woven microfibers layer (5), the thickness of the layer (6) ofnon-woven microfibers varies in dependence on the locations of the holes(14) in the layer (5) of woven microfibers such that the layer (6) ofnon-woven microfibers is thicker (16) in said holes (14) and thinner(17) on top of the yarns of the layer (5) of woven microfibers, diameterof the microfibers in the layer (6) of nonwoven microfibers is smallerthan diameter of filaments in the layer (5) of woven microfibers, withthe nonwoven microfibers accumulating in the holes (14) between theyarns of woven microfibers and blocking the holes (14), and the at leastone conducting layer (3) is disposed on the layer (6) of nonconductivenon-woven microfibers.
 2. The solar cell according to claim 1, whereinthe porous insulation substrate (4) comprises a second layer (7) ofnon-woven microfibers disposed on a second side of the sheet (5) ofwoven microfibers.
 3. The solar cell according to claim 1, wherein thesheet (5) of woven microfibers comprises yarns (10) including aplurality of filaments (11) and the diameter of said non-wovenmicrofibers is smaller than the diameter of said filaments.
 4. The solarcell according to claim 1, wherein said sheet (5) of woven microfibersis made of a glass fabric, and the fibres in said layer (6) of non-wovenmicrofibers are made of glass.
 5. The solar cell according to claim 1,wherein the thickness of said sheet (5) of woven microfibers is between4 μm and 30 μm.
 6. The solar cell according to claim 1, wherein themicrofibers in the layer (6) of non-woven microfibers have a diameterless than 4 μm.
 7. The solar cell according to claim 1, wherein saidsheet (5) of woven microfibers and the layer (6) of non-wovenmicrofibers are made of ceramic microfibers.
 8. The solar cell accordingto claim 1, wherein said layer (6) of non-woven microfibers comprisesorganic microfibers.
 9. The solar cell according to claim 1, whereinsaid layer (6) of non-woven microfibers are made of nanocellulose. 10.The solar cell according to claim 1, wherein said layer (6) of non-wovenmicrofibers comprises organic microfibers and ceramic microfibers. 11.The solar cell according to claim 1, wherein said layer (6) of non-wovenmicrofibers comprises organic microfibers and glass microfibers.
 12. Thesolar cell according to claim 5, wherein the thickness of said sheet (5)of woven microfibers is between 4 μm and 20 μm.
 13. The solar cellaccording to claim 12, wherein the thickness of said sheet (5) of wovenmicrofibers is between 4 μm and 10 μm.
 14. The solar cell according toclaim 6, wherein the microfibers in the layer (6) of non-wovenmicrofibers have a diameter less than 1 μm.
 15. The solar cell accordingto claim 14, wherein the microfibers in the layer (6) of non-wovenmicrofibers have a diameter less than 0.5 μm.
 16. The solar cellaccording to claim 1, wherein a side of the layer (6) of non-wovenmicrofibers facing away from the layer (5) of woven microfibers issmooth.
 17. The solar cell according to claim 2, wherein sides of thelayers (6, 7) of non-woven microfibers facing away from the layer (5) ofwoven microfibers are smooth.